1. Field of the Invention
The present invention relates generally to the field of solid-state imaging devices, and, more particularly, the present invention relates to an improved lens structure for a CCD imaging device and a manufacturing method therefor.
2. Description of the Related Art
In solid-state imaging devices there is a region such as a transfer register which does not directly contribute to opto-electric conversion for each pixel. Therefore the opening ratio of the actual photosensing surface of a photosensing portion to the entire pixel surface is usually 50% or less. This is an inefficient use of the incident light. To eliminate this problem and to increase overall sensitivity, an increase in the ratio of the photosensing surface to the whole pixel surface has been considered. However, increasing the ratio of the actual photosensing surface is limited from a structural point of view.
In recent years, a solid-state imaging device having a so-called micro on-chip lens has been developed. In such a device, a convex microlens is provided above the photosensing surface to converge incident light on the photosensing surface, thereby effecting efficient light convergence and improving the effective opening ratio.
FIG. 3 is a side sectional view of an essential portion of solid-state imaging device having such a convex micro on-chip lens in the related art. Referring to FIG. 3, reference numeral 1 denotes a solid-state imaging device, and reference numeral 2 denotes a semiconductor substrate, such as a silicon wafer. In a surface layer of the semiconductor substrate 2, a photosensing portion 3 for performing opto-electric conversion is formed. A reading portion 4 and a transfer register 5 are formed on one side of the photosensing portion 3, and a channel stop 6 is formed on the other side of the photosensing portion 3.
An insulating film 7 is formed on the surface of the semiconductor substrate 2, and a transfer electrode 8 for driving the transfer register 5 is formed on the insulating film 7 at a position just above the transfer register 5. An interlayer insulating film 9 is formed on the insulating film 7 and the transfer electrode 8. A light shielding film 10 is formed on the interlayer insulating film 9 so as to cover the transfer electrode 8 in order to prevent the incidence of light on the transfer register 5. The light shielding film 10 formed on the interlayer insulating film 9 does not cover a photosensing surface 3a of the photosensing portion 3. Further, an interlayer film (planarizing film) 11 of a transparent material is formed over the surface of the semiconductor substrate 2 so as to cover the photosensing surface 3a of the photosensing portion 3 and the light shielding film 10.
A color filter 12 is formed on a unit pixel consisting of these components, and a convex micro on-chip lens 13 of a transparent resin or the like is formed on the color filter 12 so as to be convex on one side opposite to the photosensing surface 3a. The micro on-chip lens 13 is located so as to focus incident light on the photosensing surface 3a of the photosensing portion 3. In the solid-state imaging device 1 having the above configuration, when light is incident on the micro on-chip lens 13 as shown by the arrows in FIG. 3, the incident light is converged on the photosensing surface 3a by the light converging effect of the micro on-chip lens 13, thereby improving the effective opening ratio in the solid-state imaging device 1.
In manufacturing the solid-state imaging device 1, the micro on-chip lens 13 in particular is obtained usually by forming a transparent resin layer of styrene resins on the color filter 12 and then dry-etching the transparent resin layer. Alternatively, the micro on-chip lens 13 is obtained by directly subjecting a transparent resin layer to a thermal reflow process or by first forming a resist layer on the transparent resin layer and then subjecting the resist layer to a thermal reflow process. Thus, the micro on-chip lens 13 is obtained directly or indirectly by utilizing a surface tension of the resin layer or the resist layer.
However, the related art solid-state imaging device has the following disadvantages. In manufacturing the solid-state imaging device 1, the micro on-chip lens 13 in particular is formed after forming the color filter 12. Accordingly, the micro on-chip lens 13 to be formed is influenced by the planarity or the like of the color filter 12, which makes it difficult to control the working accuracy and shape uniformity of the micro on-chip lens 13.
Since the micro on-chip lens 13 is formed after forming the color filter 12, film formation for the micro on-chip lens 13 at high temperatures not less than 200 degrees Celsius cannot be performed. Accordingly, there is a very limited range of materials which can be selected for forming the micro on-chip lens 13. At present, only a material having a refractive index of about 1.5 can be selected.
While the micro on-chip lens 13 is formed usually of thermosetting styrene resins, this material has a low heat resistance and a low moisture resistance. Accordingly, there is fear of age deterioration of the lens 13 formed of this material.
It is accordingly an object of the present invention to provide a solid-state imaging device and a manufacturing method therefor which can eliminate the disadvantages associated with the fabrication of the micro on-chip lens in the related art by providing a lens different from the above-mentioned related art micro on-chip lens, thereby improving light convergence efficiency.